|
CEE
680 |
|
Fall
2011 |
Water
Chemistry
Homework Set #9
Aluminum Solubility Continued
Below are the relevant chemical equilibria
for amorphous aluminum hydroxide and aluminum phosphate. These will be used in the problems below.
|
Equilibria |
Log K |
|
AlOH3(s) (amorphous) + 3H+ = Al+3 + 3 H2O |
10.8 |
|
Al+3 + H2O = AlOH+2 + H+ |
-4.97 |
|
Al+3 + 2H2O = Al(OH)2+ + 2H+ |
-9.3 |
|
Al+3 + 3H2O
= Al(OH)3(aq) + 3H+ |
-15.0 |
|
Al+3 + 4H2O = Al(OH)4- + 4H+ |
-23.0 |
|
AlPO4•2H2O(s) (variscite) = Al+3 + PO4-3 + 2H2O |
-21 |
|
H3PO4 =
H+ + H2PO4- |
-2.15 |
|
H2PO4- = H+ + HPO4-2 |
-7.2 |
|
HPO4-2 = H+ + PO4-3 |
-12.3 |
9.1 Solubility and Mixed Solids (4 points)
Prepare
a solubility diagram for Aluminum in water.
Assume that the hydroxide phase that forms is amorphous. Also assume that 0.5 mM total phosphate is
present in the system. Present the
diagram in the usual form (log C vs pH).
Outline the zones of precipitation and mark the identity of the
precipitates.
Solution
Determine
lines for all soluble Al species for amorphous aluminum hydroxide as with
previous problems of this type. Do the
same for equations based on Aluminum phosphate.
This requires that one assume a total phosphate concentration of 0.5 mM,
and consider the a3, value. Line segments are determined between the pKa’s
of the phosphate system, and smooth curved portions are drawn between them.
From
the above graph it can be seen that the total Al concentrations based on the
two solid phases intersect at about pH 8.1.
At this same pH all lines representing the same specific Al species
should also intesect. Draw a vertical
line from the intersecion of the total Al lines. Decide which solid phase controls on each
side of that line (i.e., which one is least soluble), and erase those portions
of each line that do not give the minimum solubility (i.e., that represent the
non-controlling solid).
Finally,
mark off and label the precipitation regions accordingly.
9.2. Predominance Diagram (4 points)
Prepare a predominance diagram based on
the Aluminum Hydroxide / Aluminum Phosphate equilibria in the table above. Use a soluble aluminum concentration (AlT
of 1 mM). Again, you should outline the zones of
precipitation and mark the identity of the precipitates. Outside of these zones, you should indicated
the principal soluble species.
Solution
The
predominance diagram must have pH on the x-axis and log PT (log
total phosphate) on the y-axis.
Type
A lines
|
Line |
Species |
pH |
Notes |
|
A1 |
Al+3/AlOH+2 |
4.97 |
Later
found to be under Al(OH)3 precipitate |
|
A2 |
AlOH+2/Al(OH)2+2 |
4.33 |
Doesn’t
appear as dihydroxide never predominates |
|
A3 |
Al(OH)2+/Al(OH)3o |
5.7 |
Later
found to be under Al(OH)3 precipitate |
|
A4 |
Al(OH)3o/Al(OH)4- |
8.0 |
Later
found to be under Al(OH)3 precipitate |
Type B lines for
hydroxide precipitate
B1:
Determine the equations defining the lower and upper pH boundary on aluminum
hydroxide precipitation
Equilibrium
between Al(OH)3 and Al+3 is:
pH
=4.65
assumption
is OK as this is below the A1 line. Now
we turn to the other side of the hydroxide precipitation zone.
B4:
Equilibrium between Al(OH)3 and Al(OH)4- is:
pH
= 9.2
This
is also OK as it is at a pH above that of the A4 line where the tetra hydroxide
predominates.
Type B lines for
the phosphate precipitate
|
pH for L |
<2.15 |
2.15-7.2 |
7.2-12.3 |
>12.3 |
|||
|
pH for M |
>4.97 |
4.97-5.7 |
5.7-8.0 |
>8.0 |
|||
|
Dominant Species |
Al+3 H3PO4 |
Al+3 H2PO4- |
Al(OH)2+ H2PO4- |
Al(OH)3o H2PO4- |
Al(OH)3o HPO4-2 |
Al(OH)4- HPO4-2 |
Al(OH)4- PO4-3 |
|
pH range |
<2.15 |
2.15-4.97 |
4.97-5.7 |
5.7-7.2 |
7.2-8.0 |
8.0-12.3 |
>12.3 |
|
Equ # |
B5a |
B5b |
B7b |
B8b |
B8c |
B9c |
B9d |
Next,
determine boundary between AlPO4 and Al+3. This requires separate equations below and
above pH 2.15.
First
the B5a line:
Log PT = 3.65 - 3pH
Next the B5b
line:
Log PT = 1.5 - 2pH
And
finally, we need to know the boundary between AlPO4 and Al(OH)4-,
all of which falls between the second and third pKa where HPO4-2
is the dominant phosphate species:
So
the B9c line is:
Log
PT = -28.7 + 3pH
Type C lines:
Next
determine boundary between Al(OH)3 and AlPO4. This must be evaluated from pH 4.65 to 7.2,
and then from 7.2 to 9.2
The
C1b line is:
Log
PT = -12.3 + pH
The
C1c line is:
Log
PT = -19.5 + 2pH
This
gives us the following predominance diagram.
